Pyruvate decarboxylases were purified and partially characterised for the first time from aerobically grown Rhizopus oryzae mycelium. Two very similar pyruvate decarboxylase isoenzymes were partially separated from each other by a purification protocol including ammonium sulphate precipitation, gel filtration chromatography and anion exchange chromatography. The subunit molecular weights of the enzymes were estimated to be 61 kDa for both. The pI values determined by two-dimensional electrophoretic analysis were 5.94 and 5.82 for the isoenzymes. The isoenzymes were similar in many kinetic aspects and the K,,, values were estimated as 3.9 and 4.5 mM for each of the enzymes. Both of the isozymes showed sigmoidal kinetics with a Hill coefficient of 1.8. Substrate inhibition was observed for both of the enzymes at pyruvate concentrations above 20 mM for one of the isoenzymes and 40 mM for the second isoenzyme. The enzymes were found in two different native forms; dimer and tetramer with estimated molecular weights of 120-128 and 272 kDa with the dimer being the predominant form. The optimum pH was between 6.3 and 6.5 for both. (c) 2006 Elsevier Inc. All rights reserved.